Nose, definately; I have no problems drawing the other features...But it's definately the alignment and position of the nose that screws me up (if I'm working from imagination -- photo reference is no prob)

Depends alot on the person/subject. I find the hardest thing is the persons most dominate feature. If you get this wrong, trying to get everything else right is an uphill battle. It doesn't make much difference if you get the eyebrows perfectly right if you mess up the eyes that are a really strong feature in the image.